This invention relates to an information recording medium and to a method of manufacturing a resinous substrate to be employed for the information recording medium. In particular, this invention relates to a surface recording/reproducing type information recording medium and to a method of manufacturing a resinous substrate to be employed for such an information recording medium.
An ordinary optical disk such as CD, CD-ROM, etc. is constructed such that emboss pits are formed in conformity with the recorded data on one of the surfaces of a transparent substrate having a thickness of 1.2 mm, the emboss pits being covered thereon by a reflective film made of Al for example. The information recorded in the CD constructed in this manner can be reproduced by irradiating a converging beam onto the emboss pits from a surface of the transparent substrate which is opposite to the other surface where the reflective film is formed.
On the other hand, an optical disk such as DVD, DVD-ROM where the recording density is highly enhanced is construct ed such that finer emboss pits than those of the CD are formed on one of the surfaces of a transparent substrate having a thickness of 0.6 mm, the emboss pits being also covered thereon by a reflective film made of Al for example. The information recorded on the recording surface of the disk constructed in this manner can be reproduced in the same manner as that of the CD, i.e. by irradiating a converging beam onto the emboss pits from a surface of the transparent substrate which is opposite to the other surface where the reflective film is formed.
As for the material for the substrate having a thickness of 0.6 mm, PC (polycarbonate) which is a transparent resin is generally employed. This PC substrate having a thickness of 0.6 mm however is not sufficient in mechanical property, resulting in the warping of the substrate as it is employed singly. Therefore, in order to prevent the substrate from being warped, a couple of PC substrates each having a thickness of 0.6 mm are superimposed each other with the recording surface being directed inside, thus forming a disk having a total thickness of 1.2 mm, thereby ensuring the mechanical property thereof.
The reason for setting the substrate of DVD to 0.6 mm is to secure the tilt margin of the disk. When the density of track pitch or pit is increased, the margin of the inclination or so-called tilt of the disk is caused to decrease. Although it may be possible to secure the tilt margin by decreasing the thickness of the substrate from 1.2 mm to 0.6 mm, it will inevitably result in a deterioration of the mechanical property thereof.
Under the circumstances, there has been proposed, with a view to secure the mechanical strength of the disk while decreasing the thickness of the substrate, an idea of thickening the central portion of the disk thereby to ensure the mechanical strength thereof (Japanese Patent Unexamined Publication H9-204686). However, it is required, for ensuring a sufficient mechanical strength, to make the thickness of the signal recording region of the substrate at least 0.6 mm. Further, there is also reported an idea of making the thickness of the substrate to range from 0.1 mm to 0.6 mm (Japanese Patent Unexamined Publication H9-204688). However, the thickness of a protective substrate for sustaining the recording film as well as the film thickness of the reflective film are not referred to in the idea, thus making it difficult to practice in the actual application thereof.
U.S. Pat. No. 5,757,733 teaches an information recording medium comprising a covering layer formed on the light beam incident side, and a flat substrate sustaining a recording film. However, this covering layer is simply referred to as having a thickness of 0.6 to 1.0 mm.
For the purpose of further increasing the capacity of the disk by compacting the recording density, it is more effective, in view of assuring the tilt margin, to make the thickness of the substrate as thin as possible. However, when the thickness of the substrate becomes less than 0.6 mm, it becomes difficult to secure the mechanical strength thereof even if a couple of substrates are superimposed each other.
Moreover, this superimposition of a couple of substrates is accompanied with the problems that it not only requires the employment of an adhesive but also makes the manufacturing process thereof more complicated.
Therefore, the object of the present invention is to provide an information recording medium which is capable of securing a sufficient tilt margin and a sufficient mechanical strength even if the recording density is further increased.
Another object of the present invention is to provide a method of manufacturing a resinous substrate which is adapted to be employed for such an information recording medium.
Namely, according to this invention, there is provided an information recording medium, which comprises a substrate having a recording surface provided with emboss pits or guiding grooves; a reflective film formed on the recording surface of the substrate; and a first protective film formed on the reflective film;
wherein both sides of the information recording medium are constituted by a first surface constituting an uppermost surface on the first protective film and by a second surface formed opposite to the first surface;
the first surface is constituted as a light incident surface, thereby allowing an irradiated light beam to enter and reflect through the first surface, a recorded information being reproduced based on changes in light intensity of the reflected light beam; and wherein
a distance between the recording surface of the substrate and the light incident surface is smaller than a thickness of the substrate, and a surface roughness xe2x80x9cRxe2x80x9d of the light incident surface meets a relationship represented by the following formula (1):
Rxe2x89xa6xcex/(8n)xe2x80x83xe2x80x83(1)
where xcex is a wavelength of the light beam; and n is a refractive index of the first protective film to a light having the wavelength xcex.
This invention further provides an information recording medium, which comprises;
a substrate having a couple of recording surfaces facing to each other and each provided with emboss pits or guiding grooves;
a couple of reflective films each formed on each of the recording surfaces of the substrate;
a couple of first protective films each formed on each of the reflective film; and
a couple of light incident surfaces each constituting an outermost surface;
wherein a light beam to be irradiated is designed to be entered and reflected through the couple of light incident surfaces, a recorded information being reproduced based on changes in light intensity of the reflected light beam;
a distance between one of the light incident surfaces of the information recording medium to the other is not more than 1.2 mm;
a distance between the recording surface of the substrate and the light incident surface formed over the recording surface is smaller than a thickness of the substrate; and wherein
a surface roughness xe2x80x9cRxe2x80x9d of the couple of light incident surfaces meets a relationship represented by the following formula (1):
Rxe2x89xa6xcex/(8n)xe2x80x83xe2x80x83(1)
where xcex is a wavelength of the light beam; and n is a refractive index of the first protective film to a light having the wavelength xcex.
This invention further provides an information recording medium, which comprises;
a surface substrate having a light incident surface and an emboss pit surface provided with emboss pits and facing the light incident surface;
a first reflective film formed on the emboss pit surface of the surface substrate;
a supporting substrate having an emboss pit surface provided with emboss pits and a substrate surface facing the emboss pit surface;
a second reflective film formed on the emboss pit surface of the supporting substrate; and
a transparent adhesive layer bonding the first reflective film and the second reflective film;
wherein a first record surface is constituted by the emboss spit surface of the surface substrate and the first reflective film;
a second record surface is constituted by the emboss spit surface of the supporting substrate and the second reflective film; and wherein
a light beam to be irradiated is designed to be entered and reflected through the light incident surface, a recorded information being reproduced based on changes in light intensity of the reflected light beam.
Further, this invention also provides a method of manufacturing a resinous substrate having a first recording surface provided with emboss pits or guiding grooves, and a second recording surface facing the first recording surface and provided with emboss pits or guiding grooves, which comprises the steps of;
mounting a first stamper platen for forming a first recording surface on a first die;
mounting a second stamper platen for forming a second recording surface on a second die;
positioning the first die and second die so as to keep a space therebetween and to arrange the first stamper platen to face the second stamper platen;
filling the space formed between the first die and second die with a heated and fluidized resinous material;
bringing the first die and second die close to each other thereby adjusting an interval between the first die and second die to a predetermined distance; and
allowing the resinous material filled between the first die and second die to cool and solidify thereby to obtain a double surface substrate having a thickness of not more than 1.2 mm.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.